The increasing use of non-dispatchable sources of renewable energy such as wind, wave and solar is driving a growing demand for large- and medium-scale energy storage technologies.¹ All-vanadium flow batteries (VFBs), are a promising technology for such applications.^2-8 VFBs have a major advantage over other flow batteries in that issues arising from cross-contamination due to transport through the separating membrane is effectively eliminated because the anolyte and catholyte differ only in the oxidation state of the vanadium.

Transport of electrolyte species across the cell membrane can occur during operation of flow batteries. Water transfer between two half cells can lead to irreversible precipitation of V^V in the positive half-cell due to the increased concentration of the electrolyte.^8,9 A common practice in the operation of a VFB is periodic mixing of the catholyte and anolyte to “reset” concentrations that have changed as a result of such transfer. There have been a number of studies of vanadium transfer across various membranes, particularly with regard to its effect on coulombic efficiency.^9-12 Water transfer can occur during current flow due to electroosmosis and the electroosmotic coefficient of water during hydrogen ion drift in membranes has been the subject of several studies. However, water transfer can also occur even in the absence of current flow due to the difference in the chemical potential of water between the catholyte and the anolyte. The direction of water transfer depends on the direction of this gradient in chemical potential. In the case where there is no difference in chemical potential of water between the solutions on either side of the membrane, there is no driving force for water transfer in the absence of an electric field.

In this paper, we attempt to quantify the water affinity of various vanadium electrolytes by finding the corresponding concentrations of H₂SO₄ for which there is no net transfer of water across a Nafion membrane.

Experiments were carried out in a thermostatted, H-shaped glass apparatus consisting of two burettes joined by a cross-tube which had a barrier of Nafion membrane. In a typical experiment, a vanadium electrolyte was placed in one burette and a solution of H₂SO₄ of known concentration was placed in the other. As water transferred across the membrane, the liquid level increased on one side and decreased on the other. The volume on each side was carefully monitored as a function of time. Typical results are shown in Figure 1. It can be seen that the volume of the V^V electrolyte increases with time when the H₂SO₄ concentration on the other side of the membrane is 4.7, 4.8 or 4.9 mol dm^-3 but decreases when it is 5.1, 5.2 or 5.3 mol dm^-3. The rate of transfer of water across the membrane can be obtained from the slope of the line in each case. A plot of transfer rate versus H₂SO₄ concentration crosses the concentration axis at ~ 5.0 mol dm^-3, indicating that this concentration of H₂SO₄ has the same affinity for water as the V^V electrolyte. Similar experiments were carried out for a range of electrolytes. Results will be presented for V^V, V^IV, V^III and V^II electrolytes and their relative affinities for water will be discussed.

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